Treatment of aluminum-magnesium alloy



United States Patent 3,232,796 TREATMENT OF ALUMINUM-MAGNESIUM ALLOYWilliam A. Anderson, Verona, Pa., assignor to Aluminum Company ofAmerica, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.Filed Mar. 21, 1962, Ser. No. 181,432 14 Claims. (Cl. 148--12.7)

This application is a continuation-impart of my application Serial No.154,868, filed November 24, 1961, now abandoned.

This invention relates to improving the resistance to stress corrosionof certain aluminum-magnesium alloys in the cold worked condition.

Aluminum-magnesium alloys which are adapted to being worked generallycontain less than 8% by weight of magnesium, and often minor amounts ofother elements. This type does not respond to solution and precipitationhardening treatments to a sufficient degree to effect an increase instrength to warrant the cost thereof and hence reliance is placed uponcold working as a means of developing a strength above that of a castingor annealed wrought product. While these alloys have given satisfactoryservice in many instances it has been found that under some severecorrosive conditions, those which are in the cold worked or strainhardened temper do not possess adequate resistance to stress corrosion.The term stress corrosion as here employed refers to the combined effectof corrosion and sustained high tensile stresses, and includes thecondition where corrosion is accelerated by stress. The stresses whichare considered here are of external origin as distinguished from thoseexisting within the metal body although the latter may be a contributingfactor in the end result.

It is well known that a relatively large amount of magnesium is solublein solid aluminum, under equilibrium conditions and that the commercialaluminum-magnesium alloys in the as-fabricated condition generallycontain more magnesium in solution at room temperature than indicated onthe constitutional diagram of the aluminummagnesium system. Suchsuper-saturation may be the result of chilling conditions existingduring casting of the ingot and may be further aided by the intermediateannealing operations incident to the production of wrought articles. Ifthe alloys in that condition are subsequently cold worked it has beenobserved that there is a distinct tendency for the magnesium-containingconstituent to precipitate at room temperature over a period of time andthat such a precipitate lowers the resistance to stress corrosion. Theprecipitate is generally concentrated at the grain boundaries andconsequently corrosion occurs under corrosive conditions because of adifference in solution potential between the grain boundary precipitateand the body of the grain. This invention is directed to altering thedistribution of any precipitate and, more particularly, minimizing theconcentration of it at the grain boundaries.

It is an object of this invention to provide a method of treatingaluminum-magnesium alloy articles which materially improves theirresistance to stress corrosion.

Another object is to provide a method of making a cold workedaluminum-magnesium alloy article which possesses a high resistance tostress corrosion.

Still another object is to provide a cold worked article ofaluminum-magnesium alloy which has a high resistance to stresscorrosion.

These and other objects and advantages will become apparent from thefollowing description and examples.

I have discovered that aluminum-magnesium alloy articles can be made,which in the cold worked condition not only possess the strength whichhas characterized such articles in the past, but which are substantiallyfree from stress corrosion. This result is accomplished through asequence of working and heating steps as more particularly describedbelow. From the standpoint of the internal structure of the article, thealuminum-magnesium constituent which is present in the form of finelydivided particles of microscopic size is uniformly distributedthroughout the metal body and within the grains or grain fragmentsrather than being localized at the grain boundaries. The several stepsin my process and their sequence are shown in the accompanying flowdiagram.

My invention is applicable to those aluimnum-magnesium alloys whichcontain from 4 to 8% by weight of magnesuim which exceeds the amountthat is soluble in aluminum at room temperature under equilibriumconditions. At least 4% magnesium must be present to provide the desiredlevel of strength but more than 8% makes working too difficult to beeconomical from a commercial standpoint. In the preferred practice of myinvention the magnesium content of the alloys should be between 4.5 and7%. While the binary alloy can be used, it is frequently desirable toinclude small amounts of other elements, for example, an element of thegroup composed of 0.1 to 1% manganese and 0.1 to 0.25% chromium. Zincmay be included in amounts of 0.1 to 1%. To refine the grain size of thealloy at the ingot stage it may be advisable to add from 0.05 up to 0.2%titanium. Also, to minimize oxidation of the molten metal it is helpfulto add from 0.001 to 0.05% beryllium to the alloy. Up to 0.2% copper, upto 0.5% iron and up to 0.5% silicon can be tolerated as impurities.

The alloy may be melted, cast in ingot form and hot worked in accordancewith conventional practices employed in the art. More specifically, theingots should be heated to between 800 and 1050 F. and then hot worked.Generally, rolling is the operation best adapted to produce stock forcold working but instead it may be forged, extruded, pressed orotherwise deformed by pressure. It is essential in any case that the hotworking be carried only far enough to allow at least a further reductionin cross section of 30% by cold working. A greater reduction by coldworking is generally preferred, however.

Before the hot Worked product is cold worked, it is usually advisable tosubject the product to an intermediate annealing treatment to remove anywork hardening strains introduced by the hot working operation. Theintermediate annealing generally involves heating the worked article tobetween 600 and 800 F. and holding within that temperature range for oneto four hours. This treatment along with the preheating the ingotpreparatory to hot working causes a substantial solution of anyundissolved particles of aluminum-magnesium constituent. Moreover, therate of cooling from the treating temperatures is usually suflicientlyrapid to retain a large part of the con stituent in solid solution.Neither the hot. working operation mentioned above nor the cold workingdescribed be low substantially alters the amount of constituent held insolution at the conclusion of the working operation. It is important inany case that at least a major portion of the magnesium be in solution.The thermal treatment incident to working the alloy articles generallyproduces solution of undissolved particles so that a separate solutiontreatment is not required. It is essential, however, that the alloy bodyreceive some form of thermal treatment at a high enough temperature todissolve at least a part of any undissolved magnesium-containingconstituent.

The hot worked product, which may or may not have received anintermediate annealing treatment, is cold worked with a reduction incross section of at least 20%. The cold working may be done by rolling,drawing, pressing or any of the other methods adapted to deform themetal and produce strain hardening. It is to be understood that inreferring to cold work that the term as used herein not only encompassesstrain hardening at room temperature but also strain hardening atsomewhat elevated temperatures where what is known as equivalent coldworking is produced. Thus the alloys may be deformed at temperatures upto 500 F. and still obtain the strain hardening which is essential tothe process of the invention. If the reduction in cross section is lessthan 20%, there is inadequate strain hardening and fragmentation of thegrains, particularly if the product has been subjected to anintermediate annealing treatment.

Larger amounts of cold work are preferred in order to obtain furtherfragmentation of the grains, but there is no upper limit other than thatimposed by the dimensions of the finished product.

The foregoing cold worked article is next subjected to thermal treatmentto produce the desired precipitation of the aluminum-magnesiumconstituent to improve the resistance to stress corrosion. The treatmentconsists of heating the article to a temperature between 400 and 525 F.for a period of from 2 to 24 hours. The temperature employed is relatedto the magnesium content, the relationship being a substantially directone. Thus, alloys containing 4% magnesium should be treated at about 400F. while the alloys containing the maximum amount of magnesium, i.e. 8%,should be treated at about 525 F. and those having between 4 and 8%magnesium are heated to intermediate temperatures as determined by themagnesium content. The temperature range is critical in that attemperatures below 400 F. the precipitate tends to concentrate at thegrain boundaries instead of being uniformly distributed. The uppertemperature limit will in any case be determined by the magnesiumcontent of the alloy in relationship to the production of a uniformprecipitate. Thus, in the case of an alloy containing 8% magnesium, theprecipitation temperature should not exceed 525 F. Although it may notbe possible to eifect complete precipitation of all the dissolvedmagnesium in excess of that which is soluble at the temperature oftreatment, nevertheless a major portion of the amount which can beprecipitated is taken out of solution. The period of treatment isrelated to the degree of precipitation desired, a longer periodgenerally being used where as nearly complete precipitation as possibleat a given temperature is desired and where the temperature is in thelower portion of the range.

The article which has received the precipitation treatment is again coldworked, with a reduction of at least 10%. It is the cold working at thisstage which imparts the work hardening and resultant increase instrength which is desired since the precipitation treatment relieves atleast some of the previous work hardening strains. To achieve thehighest strength the reduction in cross section should be in theneighborhood of 75 to 80%.

To stabilize the strength of the cold worked article it may be given afurther thermal treatment at a relatively low temperature which slightlyreduces the strength but effectively deters any age softening, a changethat often occurs in cold worked aluminum-magnesium alloys. Thetreatment for articles of the type described above should consist ofheating to 150 to 350 F. for 0.5 to 10 hours.

The cold worked alloy articles produced in the foregoing manner show auniformly distributed precipitate throughout the alloy, there being noconcentration at the grain boundaries. These articles have shown anexceptionally high resistance to stress corrosion, and in many caseshave been found to be free from stress corrosion.

While the strength of the articles of course varies with the magnesiumcontent and extent of the final cold working step I have obtainedtensile strengths within the range of 55,000 to 70,000 p.s.i., yieldstrengths of 35,000 to 55,000 p.s.i. and elongation values of 10 toThese tensile properties compare favorably with those of the same orsimilar alloys which have been cold worked in the usual manner.

My invention is illustrated in the following examples.

Example 1 An alloy consisting of aluminum, 5.50% magnesium, 0.77%manganese, 0.11% chromium and the usual impurities was melted and castby conventional practice in the form. of a slab type of ingot which wasadapted to being rolled into plate and sheet. The ingot was heated to915 F. and hot rolled to sheet having a thickness of 0.188 inch. The hotrolled sheet was annealed at 650 F. which removed any previous workhardening strains and then cold rolled to sheet 0.113 inch in thicknesswhich represented a reduction in thickness of 40%. The cold rolledproduct was subjected to a special precipitation treatment consisting ofheating it to 450 F. and holding at that temperature for four hoursfollowing which it was further cold rolled to a thickness of 0.057 inch,which represented a reduction of 50%. To stabilize the tensileproperties the sheeet was heated to 250 F. and held at that temperaturefor four hours after which it was cooled to room temperature. Sampleswere taken from the sheet for tensile and stress corrosion tests. Theaverage tensile strength was found to be 64,200 p.s.i., the yieldstrength 52,300 p.s.i., and the elongation 10.0%. A portion of thesamples were given a sensitizing treatment to stimulate long exposure toroom temperature and the coincident precipitation of dissolvedconstituents. The sensitizing treatment consisted of heating the samplesat 212 F. for one week. The corrosion test consisted of stressingsamples of both the as-fabricated and sensitized materials underconstant deflection to a stress equivalent to of the yield strength andalternately immersing them in a 3.5% NaCl aqueous solution over a periodof 950 days. No failures occurred in any of the stressed specimens ineither the sensitized or nonsensitized condition.

Example 2 An alloy consisting of aluminum with associated impurities,6.25% magnesium, 0.51% manganese, 0.11% chromium and the usualimpurities was also melted and cast in the same manner as in thepreceding example. The ingot was heated to 920 F. and hot rolled toplate having a thickness of 0.25 inch. The plate was annealed at 750 F.and cold rolled to 0.102 inch thick sheet, again annealed at 750 F. andcold rolled to sheet 0.061 inch in thickness which represented areduction of 40% from the thickness of the annealed sheet. The sheetproduct was heated to 450 F. and held at that temperature for 12 hoursafter which it was cold rolled with a reduction of 20% to a thickness of0.049 inch. Finally, it was stabilized by heating to 250 F. for a periodof two hours. The average tensile properties of the stabilized sheetspecimens were tensile strength 56,700 p.s.i., yield strength 38,900p.s.i., and elongation 15.0%. These values are lower than those in thepreceding example because of the smaller reduction in thicknessfollowing the last intermediate anneal even though the magnesium contentwas slightly higher. A portion of the sepcimens were sensitized for thecorrosion test which in this case consisted of bending bothas-fabricated and sensitized specimens over a radius of inch andstressing the specimens by constant deflection across a 3 inch spanbetween supports. The stressed spcimens were alternately immersed in a3.5% NaCl aqueous solution over a period of 349 days. No failuresoccurred in the specimens in either condition.

Example 3 Another alloy consisting of aluminum, 4.91% magnesium, 0.48%manganese, 0.11% chromium and the usual impurities was melted and castinto an ingot as in the preceding examples. The ingot was preheated, hotrolled and cold rolled to sheet 0.061 inch in. thickness as in Example2. The cold rolled sheet was given a precipitation treatment by heatingit to 450 F. for four hours following which it was further cold rolledwith a reduction of 20%. This product was stabilized by heating it to250 F. for a period of two hours. The sheet specimens treated in thismanner were found to have an average tensile strength of 55,950 p.s.i.,a yield strength of 41,950 p.s.i. and an elongation of 12.0%. Thesevalues reflect the effect of a lower magnesium content than in Example2. The corrosion test employed consisted of stressing tensil testspecimens in the as-fabricated and sensitized conditions by constantdeflection to a stress equivalent to 75% of the yield strength andalternately immersing them in a 3.5% NaCl aqueous solution over a periodof 166 days. No failures occurred in specimens in either theas-fabricated or sensitized condition.

Example 4 The performance of an alloy similar to that in Example 1 whichdid not receive the special precipitation treatment is shown in thefollowing test. The alloy consisted of aluminum, 5.33% magnesium, 0.80%manganese, 0.11% chromium and the usual impurities and was cast in thesame manner as the alloy in Example 1. The ingot was preheated, hotrolled, annealed and cold rolled according to the same schedule as thatfollowed in Examples 2 and 3 which resulted in a sheet 0.061 inch inthickness. The cold rolled sheet was stabilized by heating it for fourhours at 250 F. The sheet treated in that manner had a tensile strengthof 59,200 p.s.i., a yield strength of 45,400 p.s.i. and an elongation of11.5% which reflects the effect of a smaller reduction in thicknessafter the last intermediate anneal than in Example 1. The resistance tostress corrosion was tested by exposing specimens in the stabilized andsensitized conditions to alternate immersion in a 3.5% NaCl aqueoussolution where the specimens were placed under a stress equivalent to75% of the yield strength. The stabilized speci mens failed in 46 dayswhile those that had been sensitized failed within 7 days thus amplydemonstrating the benefit derived from the percipitation treatment.

Having thus described my invention and certain examples thereof, Iclaim:

1. The method of improving the resistance to stress corrosion of coldworked aluminum-magnesium alloys consisting essentially of aluminum and4 to 8% by weight of magnesium comprising the steps of hot working apreheated body of said alloy having at least a major portion of themagnesium in solution, and thereafter cold working said worked productwith a reduction in cross section of at least 20%, heating said coldworked product to a temperature within the range of 400 to 525 F. andholding it within that temperature range for a period of 2 to 24 hourswhereby a substantially uniformly distributed precipitate ofaluminum-magnesium constituent is produced, cooling the so-treatedproduct to room temperature and cold working it with a reduction incross section of at least 2. The method according to claim 1 wherein thetemperature within the precipitation temperature range of 400 to 525 F.is substantially directly related to the magnesium content of the alloy,such that as the magnesium content increases from 4 to 8%, thetemperature of the precipitation treatment increases in the sameproportion between 400 and 525 F.

3. The method according to claim 1 wherein the tensile properties offinal cold worked product are stabilized by heating it to a temperaturebetween 150 and 350 F. for a period of 0.5 to 10 hours.

4. The method according to claim 1 wherein the alloy contains from 4.5to 7% magnesium.

5. The method according to claim 1 wherein the alloy also contains atleast one of the elements of the group composed of 0.1 to 1% manganeseand 0.1 to 0.25% chromium.

6. The method according to claim 1 wherein the alloy also contains 0.1to 1% zinc.

7. The method according to claim 1 wherein the alloy also contains from0.05 up to 0.2% titanium.

8. The method according to claim 1 wherein the alloy also contains from0.001 to 0.05% beryllium.

9. The method according to claim 1 wherein the hot worked product isannealed at 600 to 800 F. before it is cold worked in thefirst-mentioned cold working step.

10. The method of improving the resistance to stress corrosion of coldworked aluminum-magnesium alloys consisting essentially of aluminum and4 to 8% by weight of magnesium comprising heating a body of said alloyto a high enough temperature to produce solution of undissolvedmagnesium, shaping said body by hot deformation thereof, cold workingsaid body with a reduction in cross section of at least 20%, heatingsaid cold Worked product to a temperature within the range of 400 to 525F. for a period of 2 to 24 hours whereby a uniformly distributedprecipitate of aluminum-magnesium constituent is produced, cooling toroom temperature and cold working with a reduction in cross section ofat least 10%.

11. The method according to claim 10 wherein the tensile properties ofthe final cold worked product are stabilized by heating to a temperaturebetween and 350 F. for aperiod of 0.5 to 10 hours.

12. The method according to claim 10 wherein the first-mentioned heatingstep consists of heating the alloy body to between 800 and 1050 F.

13. The method according to claim 10 wherein the hot deformed product isannealed at 600 to 800 F. before it is cold worked in thefirst-mentioned cold working step.

14. The method of improving the resistance to stress corrosion of coldworked aluminum-magnesium alloys consisting essentially of aluminum and4 to 8% by weight of magnesium comprising the steps of preheating a bodyof the alloy to a temperature within the range of 800 to 1050 F. untilat least a major portion of the magnesium is in solution, hot workingsaid preheated body, annealing said hot worked product at 600 to 800 F.,cold working the annealed product with a reduction in the cross sectionof at least 20%, reheating the cold worked product to a temperaturewithin the range of 400 to 525 F. for a period of 2 to 24 hours wherebya uniformly distributed precipitate of aluminum-magnesium constituent isproduced, cooling to room temperature and cold working with a reductionin cross section of at least 10%.

References Cited by the Examiner UNITED STATES PATENTS 1,926,057 9/1933Nock et a1. 148-115 2,063,022 12/1936 Beck 148-115 2,841,512 7/1958Cooper 148-115 3,031,299 4/1962 Criner 148-115 DAVID L. RECK, PrimaryExaminer.

RAY K. WINDHAM, HYLAND BIZOT, Examiners.

1. THE METHOD OF IMPROVING THE RESISTANCE TO STRESS CORROSION OF COLDWORKED ALUMINUM-MAGNESIUM ALLOYS CONSISTING ESSENTIALLY OF ALUMINUM AND4 TO 8% BY WEIGHT OF MAGNESIUM COMPRISING THE STEPS OF HOT WORKING APREHEATED BODY OF SAID ALLOY HAVING AT LEAST A MAJOR PORTION OF THEMAGNESIUM IN SOLUTION, AND THEREAFTER COLD WEORKING SAID WORKED PRODUCTWITH A REDUCTION IN CROSS SECTION OF AT LEAST 20%, HEATING SAID COLDWORKED PRODUCT TO A TEMPERATURE WITHIN THE RANGE OF 400 TO 525* F. ANDHOLDING IT WITHIN THAT TEMPERATURE RANGE FOR A PERIOD OF 2 TO 24 HOURSWHEREBY A SUBSTANTIALLY UNIFORMLY DISTRIBUTED PRECIPITATE OFALUMINUM-MAGNESIUM CONSTITUENT IS PRODUCED, COOLING THE SO-TREATEDPRODUCT TO ROOM TEMPERATURE AND COLD WORKING IT WITH A REDUCTION INCROSS SECTION OF AT LEAST 10%.